In recent years, in conjunction with a trend of high-density mounting of semiconductor devices, the wavelength of a light source used in a light-exposure apparatus employed in photolithography—one of micro-processing method—has been shortened. Recently, application of a KrF excimer laser beam (248.4 nm) has been investigated. Thus, resist material to be irradiated with a KrF excimer laser beam serving as a light source is required to possess high sensitivity to the exposed light so as to cause reaction.
In connection with this, a method using chemically amplified resist material is proposed (Polymn. Eng. Sci., Vol. 23, 1012 (1983)). According to the method, high sensitivity is attained by incorporating, into resist material, a compound which generates an acid through exposure to light.
Examples of the compound which generates an acid through exposure to light (hereinafter called an acid generator) and incorporated into the aforementioned chemically amplified resist material include onium salts such as diazoniumdiaryl salts, alkyliodoniumtriaryl salts, and alkylsulfonium salts, which are disclosed in a number of publications (e.g., in U.S. Pat. Nos. 4,491,628 and 4,603,101, Japanese Patent Publication (kokoku) No. 2-27660, and Japanese Patent Application Laid-Open (kokai) No. 62-1140).
Compounds such as 6-dinitrobenzyl tosylate are also disclosed as acid generators (e.g., F. I. Houliban et al., Advances in Resist Technology and Processing, SPIE, Vol. 920, 67, (1988)).
Among these acid generators, onium salts such as iodonium salts and sulfonium salts, inter alia, onium salt derivatives containing sulfonate as an anion, are widely used in view of their high stability of solution during storage.
There have been known methods for producing such onium salt derivatives, including salt exchange between an onium salt derivative containing a halide ion as an anion, and sulfonic acid, silver sulfonate, or alkali metal sulfonate (e.g., J. Polymer Sci., Symposium No. 56, p. 383 (1976) and J. Rad. Curing, Vol. 4, p. 2 (1977)).
However, the above methods have drawbacks. Since exchange reaction between such an onium salt derivative and sulfonic acid or alkali metal sulfonate attains an equilibrium state, sulfonic acid or alkali metal sulfonate must be used in an amount of 1.5 mol-eq. or more based on the onium salt derivative containing a halide ion as an anion. In addition, migration of halide ions into the produced onium salt derivative containing sulfonate as an anion occurs. Furthermore, when a silver salt is used in the methods, cost for carrying out the reaction increases due to the high price of the silver salt.
Sulfonic acids which form these sulfonium salts serving as acid generators include a variety of species in terms of the substituent; from aromatic group to alkyl group. Therefore, it is preferable to obtain the sulfonic acids from a single common specific intermediate through anion exchange. However, since all sulfonic acids have an acidity stronger than that of sulfuric acid, sulfonate anions derived from the starting material unavoidably remain during a typical salt exchange process, due to equilibrium of the reaction, to thereby affect a resist containing the acid generators.
From another aspect, onium salt derivatives containing sulfonate as an anion are employed, for example, as thermal-polymerization or photo-polymerization initiators for a variety of anion-polymerizable compounds such as epoxy compounds and as photo-acid generators of chemically amplified resists.
In view of the foregoing, an object of the present invention is to provide a high-yield method for producing an onium salt derivative useful as an agent such as an acid generator used in chemically amplified resists. Another object of the invention is to provide a novel onium salt derivative.
The present inventors have carried out extensive studies in order to solve the aforementioned drawbacks, and have found that reacting an onium salt derivative containing a halide anion or a carboxylate anion with a sulfonic acid ester derivative or a phosphoric acid derivative can produce an onium sulfonate derivative or an onium phosphate derivative at high yield; that the reaction can provide a novel onium salt derivative containing phosphate as an anion; and that an onium salt derivative containing sulfonate as an anion can effectively be produced from the novel onium phosphate derivative. The present invention has been accomplished on the basis of these findings.
The present inventors have also found that a similar onium salt derivative containing sulfonate as an anion can be produced by reacting, in the presence of a specific compound such as an ortho acid ester, an onium salt derivative containing a halide anion or a carboxylate anion with a sulfonate salt. The present invention has been accomplished also on the basis of this finding.
Furthermore, the present inventors have also found that an onium sulfonate derivative can be produced at high yield by reacting an onium salt derivative containing a halide anion or a carboxylate anion with a sulfonic acid derivative such as dialkylsulfuric acid and, subsequently, with a sulfonate salt. The present invention has been accomplished also on the basis of this finding.